There has been spread of various types of optical discs, such as a DVD and a CD (Compact Disc). Moreover, there has been spread of a disc drive which is integrally provided with a function for information recording or reading with respect to a plurality types of optical discs. Such a disc drive is referred to as a multidisc drive or a multi drive. As an optical pickup of this type of disc drive, an optical disc having such a structure that it can be shared among a plurality of optical discs is sometimes used. One of these optical pickups is an optical disc for DVD/CD. This optical pickup is provided with a light source which outputs a light beam for information recording or reading with respect to a DVD and a light source which outputs a light beam for information recording or reading with respect to a CD. This optical pickup also unifies these light beams on a single optical axis and focuses it on the recording surface of the DVD or the recording surface of the CD by using a single objective lens.
By the way, the DVD has a higher recording density than that of the CD. In order to ensure stability or accuracy in the information recording or reading with respect to the DVD, it is desirable to provide the optical pickup with a mechanism for correcting spherical aberration, coma aberration, and astigmatism aberration. Some of the optical pickups for DVD/CD which are currently popular are provided with an aberration correcting apparatus which uses a liquid crystal (LC) to correct the spherical aberration, the coma aberration, and the astigmatism aberration with regard to the DVD. The aberration correcting apparatus is provided, for example, with a LC optical element and a LC driver for driving the LC optical element. The LC optical element is normally located on the incident side of the objective lens and is disposed to cross the optical axis. The LC optical element has two electrode layers which are arranged to face each other with the LC therebetween. Each of the electrode layers has a transparent electrode pattern formed to correct the spherical aberration, the coma aberration, and the astigmatism aberration with regard to the DVD. When correcting the aberration, it applies voltages to electrodes, changes LC orientation, and changes LC refractive index. By this, an optical path length (or phase) of the light beam which passes through the LC is changed, to thereby uniform the wavefront of the light beam for DVD and correct the aberration.
On the other hand, the CD has a lower recording density than that of the DVD. Moreover, a signal processing technique about the information recording or reading with respect to the CD is sufficiently mature. From such situations, it has been generally considered that it is only necessary to correct the coma aberration on the optical pickup, with regard to the CD. In fact, the aberration correcting apparatus provided for the optical pickups for DVD/CD which are currently popular has a function for correcting only the coma aberration with regard to the CD although it has a function for correcting the spherical aberration, the coma aberration, and the astigmatism aberration with regard to the DVD.